Ac powered logic circuitry

ABSTRACT

The use of an alternating current (ac) source to power logic circuitry can support satisfactory device performance for a variety of applications, while enhancing long-term stability of the circuitry. For example, when organic thin film transistor (OTFT)-based logic circuitry is powered by an ac power source, the logic circuitry exhibits stable performance characteristics over an extended period of operation. Enhanced stability may permit the use of OTFT logic circuitry to form a variety of circuit devices, including inverters, oscillators, logic gates, registers and the like. Such circuit devices may find application in a variety of applications, including integrated circuits, printed circuit boards, flat panel displays, smart cards, cell phones, and RFID tags. In some applications, the ac-powered logic circuitry may eliminate the need for ac-dc rectification components, thereby reducing the manufacturing time, expense, cost, complexity, and size of the component carrying the circuitry.

STATEMENT OF PRIORITY

This application is a divisional of U.S. Ser. No. 10/328,461 filed onDec. 23, 2002, now allowed, the disclosure of which is herebyincorporated by reference.

FIELD

The invention relates to logic circuitry.

BACKGROUND

Thin film circuit devices, including transistors, diodes, and the like,are widely used in a variety of modern electronic devices, includingintegrated circuits, printed circuit boards, flat panel displays, smartcards, cell phones, and radio frequency identification (RFID) tags. Thinfilm circuit devices are typically formed by depositing, masking andetching a variety of conducting, semiconducting and insulating layers toform a thin film stack.

Typically, thin film transistors are based on inorganic semiconductormaterials such as amorphous silicon or cadmium selenide. More recently,significant research and development efforts have been directed to theuse of organic semiconductor materials to form thin film transistorcircuitry.

Organic semiconductor materials offer a number of manufacturingadvantages for transistor fabrication. In particular, organicsemiconductor materials permit the fabrication of organic thin filmtransistors (OTFTs) on flexible substrates such as thin glass, polymericor paper-based substrates. In addition, organic semiconductor materialscan be formed using low-cost fabrication techniques such as printing,embossing or shadow masking. Although the performance characteristics ofOTFTs have improved with continued research and development, deviceperformance and stability continue to present challenges.

SUMMARY

In general, the invention is directed to logic circuitry powered byalternating current (ac) power sources. The invention may be applied tologic circuitry incorporating thin film transistors based on amorphousor polycrystalline organic semiconductors, inorganic semiconductors orcombinations of both.

The use of an ac power source to power thin film transistor-based logiccircuitry can support satisfactory device performance for a variety ofapplications, while enhancing long-term stability of the circuitry. Forexample, when OTFT circuitry is powered by an ac power source, the OTFTcircuitry may exhibit stable performance characteristics over anextended period of operation.

Enhanced stability may permit the use of OTFT circuitry to form avariety of thin film transistor-based logic circuit devices, includinginverters, oscillators, logic gates, registers and the like. Such logiccircuit devices may find utility in a variety of applications, includingintegrated circuits, printed circuit boards, flat panel displays, smartcards, cell phones, and RFID tags.

For some applications, ac-powered thin film transistor circuitry mayeliminate the need for an ac to dc rectification block, thereby reducingthe manufacturing time, expense, cost, complexity, and size of thecomponent carrying the thin film transistor circuitry. The ac powersource directly powers the logic gate circuitry. In particular, the acpower source applies an ac power waveform to one or more individuallogic gates, instead of applying dc power to the logic gates via anac-dc rectification block.

In one embodiment, the invention provides an electronic circuitcomprising a first transistor and a second transistor arranged to form alogic gate, and an alternating current (ac) source to directly power thelogic gate with an ac power waveform.

In another embodiment, the invention provides a method comprisingdirectly powering a logic gate formed by at least a first transistor anda second transistor with an alternating current (ac) power waveformproduced by an alternating current (ac) power source.

In an added embodiment, the invention provides a radio frequencyidentification (RFID) tag comprising a logic gate formed by at least afirst transistor and a second transistor, and a radio frequencyconverter that converts RF energy to alternating current (ac) power, anddirectly powers the logic gate with the ac power.

In a further embodiment, the invention provides a radio frequencyidentification (RFID) system comprising an RFID tag including first andsecond transistors arranged to form a logic gate, a radio frequency (RF)converter that converts RF energy to alternating current (ac) power anddirectly powers the logic gate with the ac power, and a modulator thatconveys information, and an RFID reader that transmits the RF energy tothe RFID tag for conversion by the RF converter, and reads theinformation conveyed by the modulator.

In another embodiment, the invention provides a ring oscillator circuitcomprising a plurality of transistors arranged to form a series ofinverter stages, the inverter stages being coupled to form a ringoscillator, and an alternating current (ac) source to directly power theinverter stages in the ring oscillator with an ac power waveform.

The invention can provide a number of advantages. For example,ac-powered logic circuitry, and particularly OTFT-based logic circuitry,may exhibit increased stability over an extended period of time,relative to dc-powered thin film transistor circuitry. In the case of aring oscillator, for example, ac-powered thin film transistor circuitrymay maintain oscillation amplitudes over a longer period of timerelative to dc-powered thin film transistor circuitry.

The availability of stable OTFT circuitry, in particular, may promotewider use of OTFT circuitry in a variety of applications, with morereliable performance, durability and longevity. Consequently, variousapplications for OTFT circuitry may benefit from manufacturingadvantages associated with OTFT circuitry, such as the ability to formcircuitry on flexible substrates, such as thin glass, polymeric orpaper-based substrates, and use lower-cost manufacturing techniques.

As a further advantage, the use of ac power for the thin film transistorcircuitry may eliminate the need for the ac-dc rectifier componentotherwise required in some applications for delivery of dc power to thecircuitry. Accordingly, by eliminating the need for a rectifiercomponent, the use of ac power may reduce the manufacturing time,expense, cost, complexity, and size of components carrying thin filmtransistor circuitry.

For RFID tags, as a particular example, the use of ac-powered thin filmcircuitry may substantially reduce the cost and size of the tag byeliminating the ac-dc rectifier component. In particular, by eliminatingthe need for a front-end rectifier block, ac-powered thin film logiccircuitry can result in substantial cost and size savings in the designand manufacture of the RFID tag.

Additional details of these and other embodiments are set forth in theaccompanying drawings and the description below. Other features, objectsand advantages will become apparent from the description and drawings,and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram illustrating an ac-powered inverter circuit.

FIG. 2 is a graph illustrating simulated performance of the invertercircuit of FIG. 1.

FIG. 3 is a circuit diagram illustrating an ac-powered inverter circuitbased on complementary metal oxide semiconductor (CMOS) transistors.

FIG. 4 is a circuit diagram illustrating an ac-powered NAND gatecircuit.

FIG. 5 is a circuit diagram illustrating an ac-powered thin filmtransistor-based NOR gate circuit.

FIG. 6 is a circuit diagram illustrating an ac-powered thin filmtransistor-based ring oscillator circuit with load capacitors.

FIG. 7 is a graph illustrating simulated performance of the ringoscillator circuit of FIG. 6.

FIG. 8 is a circuit diagram illustrating an ac-powered thin filmtransistor-based ring oscillator circuit without load capacitors.

FIG. 9 is a graph illustrating simulated performance of the ringoscillator circuit of FIG. 8.

FIG. 10 is a block diagram illustrating application of ac-powered thinfilm transistor circuitry in an RFID tag/reader system.

FIG. 11 is a circuit diagram further illustrating the RFID tag/readersystem of FIG. 10.

FIG. 12 is a circuit diagram further illustrating a reader associatedwith the RFID tag/reader system of FIG. 10.

FIG. 13 is a graph illustrating simulated output of an RFID tagconstructed using ac-powered thin film transistor circuitry.

FIG. 14 is a circuit diagram illustrating an ac-powered inverter circuitthat drives a liquid crystal display element.

FIG. 15 is a circuit diagram illustrating an ac-powered inverter circuitthat drives a light emitting diode (LED).

DETAILED DESCRIPTION

FIG. 1 is a circuit diagram illustrating an ac-powered inverter circuit10. Inverter circuit 10 may include an ac power source 12 that suppliesac power to a logic gate in the form of an inverter 14 having a loadtransistor 16 and a drive transistor 18. Each transistor 16, 18 may be athin film field effect transistor, and may be based on an amorphous orpolycrystalline inorganic or organic semiconducting material. As anexample, organic semiconducting materials such as pentacene may be usedto form OTFTs. As an alternative, circuit 10 may be formed by acombination of organic and inorganic semiconducting material, e.g., toform a complementary metal oxide semiconductor (CMOS) inverter circuit.For example, in some applications, inverter circuit 10 may be formed byan NMOS inorganic field effect transistor and a PMOS organic fieldeffect transistor. When OTFTs are used, transistors 16, 18 may beespecially adaptable to fabrication using low cost fabricationtechniques, and may be formed on flexible substrates for someapplications.

The ac power source 12 directly powers inverter 14 with an ac powerwaveform. The ac power is applied directly to inverter 14 in the sensethat the inverter receives an ac power waveform instead of dc powerproduced by an ac-dc rectification component. In other words, inverter14 operates in response to an ac power waveform. Accordingly,intervening circuitry may exist between ac power source 12 and inverter14 provided that the inverter still receives an ac power waveform asoperating power, rather than a dc power signal. In the example of FIG.1, the ac power waveform is applied directly across the common gate anddrain connection of load transistor 16 and the ground connection coupledto the source of drive transistor 18.

The use of ac power source 12 to power thin film transistor-based logiccircuitry, such as inverter 14 in FIG. 1, can support satisfactorydevice performance for a variety of applications, while enhancinglong-term stability of the circuitry. For example, when inverter 14 ispowered by ac power source 12, the inverter may exhibit stableperformance characteristics over an extended period of operationrelative to dc-powered inverters, especially for organic semiconductingmaterials. Also, for some applications, ac operation of inverter 14 willeliminate the need for an ac-dc rectifier component to power theinverter. The ac power waveform supplied to inverter circuit 10 may havea variety of regular shapes, e.g., sinusoidal, square, orsawtooth-shaped. In addition, in some embodiments, the ac power waveformmay have irregular shapes. Accordingly, the ac power waveform exhibitsan alternating current flow but is not limited to any particular shape.Nevertheless, in many applications, the ac power waveform may be asinusoidal waveform.

As shown in FIG. 1, the gate and drain of load transistor 16 are coupledto ac power supply 12. The drain of drive transistor 18 is coupled tothe source of load transistor 16, and the source of the drive transistoris coupled to ground. A signal source 20 drives the gate of drivetransistor 18, e.g., with a logic signal. In response, inverter 14produces an inverted output 22, which may be output across a loadcapacitor 24. Load capacitor 24 serves to filter out some of the acvoltage present at the output and provides for a cleaner output signal.The amount of filtering depends on the capacitance of load capacitor 24and the frequency of the ac power. Load capacitor 24 may be formed by aninput capacitance produced by gate/source overlap within a logic gatecoupled to output 22 in the event inverter circuit 10 is coupled todrive one or more additional logic gates.

The gate/source overlap may be controlled during manufacture of a drivetransistor 18 in a subsequent logic gate to produce a desired level ofcapacitance in load capacitor 24. Alternatively, load capacitor 24 maybe formed independently, particularly if output 22 does not driveanother logic gate. In some embodiments, load transistor 16 may have agate width to gate length ratio that is greater than or equal to a gatewidth to gate length ratio of the drive transistor 18. In this case,direct current (dc) powering of the circuit would result in inferioroperation of the logic gate, for NMOS or PMOS designs, because of thereduced gain. NMOS or PMOS ring oscillators based on this design, forexample, would not operate if powered by direct current. An addedbenefit of having the gate width to gate length ratio of load transistor16 greater or equal to the drive transistor 18 gate width to gate lengthratio is that the total circuit area can be reduced.

FIG. 2 is a graph illustrating simulated performance of the invertercircuit 10 of FIG. 1. In particular, the graph illustrates signalvoltage transitions over a period of time. For purposes of thissimulation, transistors 16, 18 are modeled as PMOS organic field effecttransistors. In FIG. 2, trace 25 is the input signal waveform applied tothe gate of drive transistor 18 by signal source 20. Trace 26 is theoutput signal waveform produced by inverter 14 at output 22. In theexample of FIG. 2, the input signal waveform transitions between a logic‘0’ state 28 and a logic ‘1’ state 27. In response, inverter 14 producesan inverted output in response to the input signal waveform, i.e., alogic ‘1’ state 32 and a logic ‘0’ state 30, as shown in FIG. 2.Inverter 14 exhibits a propagation delay that is inversely related tothe ac voltage applied to load transistor 16 and the mobility of thesemiconductor material forming the inverter, and proportional to theparasitic capacitance within transistors 16, 18 and any externalcapacitance that may be independently added to inverter circuit 10. Theac power source 12 may have a frequency characterized by a period thatis less than the propagation delay time of inverter 14.

In the example of FIG. 2, ac power source 12 produces a sinusoidalwaveform having a frequency of 125 kHz and a peak-to-peak amplitude of80 volts. Also, signal source 20 produces a square wave input signalwaveform between approximately 0 and −15 volts, at approximately 100 Hz.Inverter 14 turns “on” in response to the ac power supply waveformapplied by ac power source 12, and serves to invert the input signalwaveform applied by signal source 20. The output 22 of inverter 14 maybe applied to additional logic circuitry. In addition, a plurality ofinverters 14 may be combined to form a variety of logic components, suchas oscillators, logic gates, registers and the like. Although invertercircuit 10 is depicted in FIG. 1 for use as a logic gate, the invertercircuit may be used as an analog amplifier in some cases. In addition,inverter circuit 10 can be used to drive a variety of loads, includingdisplay elements such as liquid crystal display (LCD) elements, or lightemitting diodes (LEDs), including organic light emitting diodes (OLEDs).

FIG. 3 is a circuit diagram illustrating an ac-powered thin filmtransistor-based inverter circuit 14′ incorporating CMOS-basedcircuitry. As shown in FIG. 3, the source of p-channel transistor 16 iscoupled to ac power supply 12. An n-channel transistor 19 has a draincoupled to the drain of transistor 16. In addition, the gates oftransistors 16, 19 are coupled together and driven by a signal source20. Signal source 20 drives the gates of transistors 16, 19, e.g., witha logic signal. The source of transistor 19 is coupled to ground. Thesource of transistor 16 and the drain of transistor 19 are coupledtogether to form the output 22 of inverter circuit 14′. In response tothe logic signal, inverter 14′ produces an inverted output 22. In someembodiments, a load capacitor may be coupled between output 22 andground. Again, the load capacitor may be formed by the input capacitanceof a subsequent logic gate coupled to the output of inverter circuit14′. Alternatively, a load capacitor may be formed independently toprovide the desired load capacitance for output 22.

FIG. 4 is a circuit diagram illustrating an ac-powered thin filmtransistor-based NAND gate circuit 21. As shown in FIG. 4, the gate anddrain of load transistor 16 are coupled to ac power supply 12. The drainof first drive transistor 18A is coupled to the source of loadtransistor 16. The drain of second drive transistor 18B is coupled tothe source of first drive transistor 18A. The source of second drivetransistor 18B is coupled to ground. First and second signal sources20A, 20B drive the gates of drive transistors 18A, 18B, respectively. Inresponse, NAND gate 23 produces a logical NAND output 22. Transistors16, 18A, 18B form a NAND gate. NAND circuit 21 is operative in responseto the ac power supply signal delivered directly to the NAND circuit byac power supply 12. In some embodiments, a load capacitor may be coupledacross output 22. The load capacitor may be formed independently orrealized by the input capacitance of a logic gate driven by output 22 ofNAND circuit 21.

FIG. 5 is a circuit diagram illustrating an ac-powered thin filmtransistor-based NOR gate circuit 25. FIG. 5 represents another exampleof a thin film transistor-based logic circuit that operates with an acpower supply, in accordance with the invention. As shown in FIG. 5, thegate and drain of load transistor 16 are coupled to ac power supply 12.Transistors 16, 29A, 29B form a NOR gate 27. The drains of first andsecond drive transistors 29A, 29B are coupled to the source of loadtransistor 16, and to output 22. The sources of first and second drivetransistors 29A, 29B are coupled to ground. First and second signalsources 31A, 31B drive the gates of drive transistors 29A, 29B,respectively. In response, NOR gate 27 produces a logical NOR output 22.NOR circuit 25 is operative in response to the ac power supply signaldelivered by ac power supply 12. In some embodiments, a load capacitormay be coupled across logical NOR output 22. The load capacitor may beformed independently or realized by the input capacitance of a logicgate driven by output 22 of NOR circuit 25.

FIG. 6 is a circuit diagram illustrating an ac-powered thin filmtransistor-based ring oscillator circuit 33. Ring oscillator circuit 33is an example of another circuit that can be implemented usingac-powered logic gates, e.g., including inverter stages based on OTFTs.As shown in FIG. 6, ring oscillator circuit 33 includes an odd number ofinverter stages arranged in series. In the example of FIG. 6, ringoscillator circuit 33 includes seven inverter stages 36A-36G having,respectively, load transistors 34A-34G and drive transistors 35A-35G,respectively. Each transistor 34, 35 in ring oscillator circuit 33 is anac-powered thin film field effect transistor. For example, ac powersource 12 delivers ac power to the gates and drains of load transistors34. The source electrodes of drive transistors 35 are coupled to ground.

In the example of FIG. 6, each inverter stage 36 has an output coupledacross a respective load capacitor 38A-38G. For example, the output ofinverter stage 36A is coupled across load capacitor 38B, and the outputof inverter stage 36G is coupled across load capacitor 38A. Eachcapacitor 38 may be formed by the input capacitance produced bygate/source overlap within a drive transistor 35 of a subsequentinverter stage 36 that is driven by the output of a respective inverterstage. The output 40 of final inverter stage 36G is coupled to the gateof drive transistor 35A in first inverter stage 36A to provide feedback.Like inverter circuit 10 of FIG. 1, ring oscillator circuit 33 of FIG. 6operates in response to the ac power supply waveform delivered by acpower supply 12. During operation, ring oscillator circuit 33 provides aclock signal. For example, the output of each inverter stage 36 in ringoscillator circuit 33 can be tapped to provide a clock signal with adesired phase.

FIG. 7 is a graph illustrating simulated performance of the ringoscillator circuit 33 of FIG. 6. As shown in FIG. 7, ring oscillator 33produces, as output 41, an oscillating output waveform 41 characterizedby high peaks 42 and low peaks 43. In the example of FIG. 7, ac powersource 12 produces a sinusoidal waveform having a frequency of 125 kHzand a peak-to-peak amplitude of 40 volts. Oscillating output waveform 41in FIG. 7 exhibits a frequency of approximately 300 Hz. In general, theoutput waveform produced by a ring oscillator circuit will have afrequency that is dependent on the number of inverter stages 36 and thepropagation delays produced by the individual inverter stages. Thepropagation delay is inversely related to the ac power supply voltageapplied to ring oscillator circuit 33 and the mobility of thesemiconducting material, and proportional to any applicable parasitic orexternal capacitance present in inverter stages 36.

FIG. 8 is a circuit diagram illustrating an ac-powered thin filmtransistor-based ring oscillator circuit 33′ without capacitors 38. FIG.9 is a graph illustrating simulated performance of the ring oscillatorcircuit 33′ of FIG. 8. Ring oscillator circuit 33′ of FIG. 8 conformssubstantially to ring oscillator circuit 33 of FIG. 6, but does notinclude capacitors 38 at the outputs of respective inverter stages 36.In the absence of capacitors 38, the oscillating output waveform 41′, inFIG. 9 including peaks 44 and 46, reveals more of the 125 KHz ac powersupply waveform.

Operation of thin film transistor circuitry, such as ring oscillatorcircuit 33, also is possible with higher ac power supply frequencies.Functioning ring oscillator circuits that conform substantially tocircuit 33 have been observed to operate, for example, with ac powersupply frequencies on the order of 6 MHz. With increased semiconductormobility, it may be reasonable to expect use of ring oscillator circuitsas described herein with ac power supply frequencies of greater than 10MHz.

FIG. 10 is a block diagram illustrating application of ac-powered thinfilm transistor-based circuitry in an RFID tag/reader system 55. Use ofac-powered thin film transistor-based circuitry may be particularlydesirable in an RFID tag for a number of reasons, as will be described.As shown in FIG. 10, system 55 may include a reader unit 56 and an RFIDtag 58.

Reader unit 56 may include a radio frequency (RF) source 62 and a reader64. RF source 62 transmits RF energy to RFID tag 58 to provide a sourceof power. In this manner, RFID tag 58 need not carry an independentpower supply, such as a battery. Instead, RFID tag 58 is powered acrossa wireless air interface between reader unit 56 and the RFID tag. Tothat end, reader unit 56 includes an inductor 59 that serves, in effect,as an antenna to transmit and receive RF energy.

As further shown in FIG. 10, RFID tag 58 may include an ac power source66. As will be explained, ac power source 66 may serve to convert RFenergy transmitted by reader unit 56 into ac power for delivery to thinfilm transistor circuitry carried by RFID tag 58. RFID tag 58 mayreceive the RF energy from reader unit 56 via an inductor 67 that servesas a receiver. A capacitor 77 also may be provided in parallel withinductor 67. RFID tag 58 further includes a clock circuit 68, datacircuit 70, control logic circuit 72, output buffer circuit 74 andmodulation inverter 76, one or more of which may be formed by anarrangement of thin film transistor circuitry.

Clock 68 drives control logic circuit 72 to output data from datacircuit 70, which may comprise a plurality of data lines carrying anidentification code. Output buffer circuit 74 buffers the output fromcontrol logic circuit 72. Modulation inverter 76, in turn, modulates thebuffered output for interpretation by reader unit 56 via inductor 67.For example, modulation inverter 76 conveys the information bymodulating the signal applied across inductor 67.

FIG. 11 is a circuit diagram further illustrating the RFID tag/readersystem 55 of FIG. 10. As shown in FIG. 11, RF source 62 may include anac generator 71 that transmits an ac output signal via inductor 59. Forsome applications, ac generator 71 may take the form of a sinusoidalcurrent source with an output of approximately 0 to 5 amps at afrequency of approximately 125 kHz.

Inductors 59 and 67 form a transformer for electromagnetic coupling ofRF energy between RF source and RFID tag 58. Resistor 73 is selected tolimit current. A capacitor 77 is placed in parallel with inductor 67within ac power source 66 to form a parallel resonant tank that governsthe frequency of the ac power source according to the equation:${f = \frac{1}{2\pi\sqrt{LC}}},$where L is the inductance of inductor 67 and C is the capacitance ofcapacitor 77. With an inductance of 50 μH and a capacitance of 32 nF,inductor 67 and capacitor 77 generate a resonant frequency ofapproximately 125 KHz. Hence, in this example, the output of ac powersource 66 is a sinusoidal waveform with a frequency of approximately 125kHz. This waveform is then applied to clock circuit 68, control logic72, data lines 70 and output buffer 74 as represented in FIG. 11 by theterminals AC POWER and COMMON.

FIG. 11 depicts an RFID tag 58 that carries an n-bit identificationcode. For ease of illustration, RFID tag 58 carries a 7-bitidentification code specified by data lines 70. In many applications,RFID tag 58 may carry a much larger identification code, e.g., 31-bit,63-bit or 127-bit codes. In some embodiments, selected data lines 70 maycarry information used for start bit identification, data streamsynchronization and error checking. In the example of FIG. 11, clockcircuit 68 is a ring oscillator formed by a series of seven inverterstages arranged in a feedback loop.

The ring oscillator of FIG. 11 may be similar to ring oscillator 33 or33′ of FIGS. 6 and 8. The outputs of two successive inverters areapplied to a respective NOR gate provided in control logic 72. In thisway, seven NOR gates are used to generate a sequence of seven pulseswithin each clock cycle produced by the ring oscillator. Note that thenumber of NOR gates in control logic 72 may vary. Again, thisarrangement could be extended, in principle, to larger numbers of bits,e.g., n=31, 63 or 127.

Switches shown in series with data lines 70 are connected to respectiveNOR gate outputs at one end. If a switch is closed, the respective dataline couples the NOR gate output to ground If the switch is open, theNOR gate output is coupled as one of the inputs to a 7-input OR gatewithin control logic 72.

In the example of FIG. 11, the switches for second and fourth data lines(from left to right) are closed. As a result, data lines 70 store the7-bit identification code “1010111.” The switches can be made, forexample, from metal lines that extend from the NOR gate outputs toground. The electrical connections to ground can be intentionally brokenor connected during manufacturing to produce, in effect, an open switch,and thereby encode a unique identification code into data lines 70 ofRFID tag 58. The electrical connections may be broken by a variety ofmanufacturing techniques such as, for example, laser etching, mechanicalscribing, electrical fusing, or shadow masking.

The output of the 7-input OR gate in control logic 72 is applied to acascade of buffer amplifiers in output buffer 74 to help match theoutput impedance of the logic circuitry to the input impedance of themodulation inverter 76. The output of the buffer amplifiers in outputbuffer 74 is applied to the input of the modulation inverter 76.Specifically, the signal TAG OUTPUT is applied to the gate of the drivetransistor associated with modulation inverter 76. Modulation inverter76 then modulates the Q of the tank formed by inductor 67 and capacitor77 to provide amplitude modulation of the carrier signal. In thismanner, the received buffer output is conveyed to reader unit 56 so thatthe identification code can be read by reader 64. In particular, reader64 processes the signal received at L_tap via inductor 59.

FIG. 12 is a circuit diagram further illustrating reader 64 associatedwith the RFID tag/reader system 55 of FIG. 10. Reader 64 receives, viaL_tap, a signal containing the carrier signal, e.g., at 125 kHz,modulated by the TAG OUPUT signal, which may be on the order of 1 kHz,depending on the frequency of clock circuit 68. A low junctioncapacitance signal diode 78 is used to demodulate the signal. A low passfilter section 80 removes the carrier frequency, and may includeinductor 84, capacitor 86, resistor 88, inductor 90, capacitor 92 andresistor 94. An amplifier stage 82 includes an amplifier 98 in anon-inverting configuration, with resistor 96 and feedback resistor 100coupled to the inverting input.

FIG. 13 is a graph illustrating simulated output of an RFID tagconstructed using ac-powered thin film transistor circuitry as shown inFIGS. 10-12. In particular, FIG. 13 shows the transition of the signalTAG OUTPUT generated from output buffer 74. As shown in FIG. 13, duringthe application of the ac power supply waveform to clock circuit 68,control logic 72, data lines 70, and output buffer 74, the circuitry isoperative to produce a train of pulses in sequence with the clockcircuit 68.

FIG. 13 shows a transition between bit 0 (102), bit 1 (104), bit 2(106), bit 3 (108), bit 4 (110), bit 5 (112) and bit 6 (114) of theidentification code specified by data lines 70. In particular, it can beseen from FIG. 13 that the 7-bit code transitions from high to low in apattern corresponding to the code 1010111. Accordingly, such a patterncan be readily resolved by reader 64 to determine the identificationcode carried by RFID tag 58.

FIG. 14 is a circuit diagram illustrating an ac-powered thin filmtransistor-based inverter circuit 116 that drives a liquid crystaldisplay element 118. In the example of FIG. 14, inverter circuit 116conforms substantially to inverter circuit 10 of FIG. 1. However, theoutput of inverter 14 drives a liquid crystal display element 118. Inparticular, one electrode of liquid crystal display element 118 iscoupled to the source of load transistor 16 and the drain of drivetransistor 18. The other electrode of liquid crystal display element 118is coupled to ground.

FIG. 15 is a circuit diagram illustrating an ac-powered thin filmtransistor-based inverter circuit 120 that drives a light emitting diode(LED) 122. Inverter circuit 120 conforms substantially to invertercircuit 10 of FIG. 1, but drives an LED 122. The cathode of LED 122 iscoupled to the source of load transistor 16 and the drain of drivetransistor 18, and the anode of the LED is coupled to ground.

The invention can provide a number of advantages. For example,ac-powered logic circuitry, and particularly OTFT-based logic circuitry,may exhibit stable performance over a longer period of time, relative todc-powered thin film circuitry. Although dc-powered OTFT logic circuitryappears to undergo substantial changes in threshold voltage over time,the overall performance of ac-powered OTFT logic circuitry does not seemto change as quickly. Instead, ac-powered OTFT circuitry seems to bemore stable over an extended period of time.

In the case of a ring oscillator, for example, ac-powered OTFT circuitryappears to maintain oscillation amplitudes over a much longer period oftime relative to dc-powered OTFT circuitry. When OTFT-based ringoscillators are powered with dc power and monitored over time, theoscillation amplitude can exhibit a rather rapid decrease. When the sametype of ring oscillator is ac powered, however, the rapid decrease doesnot occur. In particular, consistent oscillation amplitude has beenobserved for an ac-powered OTFT-based ring oscillator runningcontinuously for over sixty hours, in contrast to a dc-poweredOTFT-based ring oscillator which exhibited performance changes in lessthan ten minutes.

The availability of stable and reliable OTFT circuitry may promote wideruse of OTFT circuitry in a variety of applications, with more reliableperformance, durability and longevity. Consequently, variousapplications for ac-powered OTFT circuitry, including those describedherein, may benefit from manufacturing advantages associated with OTFTcircuitry, such as the ability to form circuitry on flexible substratesand use lower-cost manufacturing techniques.

As a further advantage, the use of ac power for the thin film circuitrymay eliminate the need for the ac-dc rectifier circuitry otherwiserequired in some applications for delivery of dc power to the circuitry.Accordingly, by eliminating the need for rectifier circuitry, the use ofac power may reduce the manufacturing time, expense, cost, complexity,and size of components carrying thin film circuitry.

For RFID tags, as a particular example, the use of ac-powered thin filmcircuitry may substantially reduce the cost and size of the circuit byeliminating the ac-dc rectifier circuitry. In addition, the RFID tag maybenefit from performance and reliability advantages associated withac-powered OTFT circuitry, possibly creating new opportunities forapplication of RFID technology. For example, the increased reliabilityof ac-powered OTFTs may permit applications in which the RFID tag, inwhatever form, is in more continuous or even persistent operation inconjunction with a reader unit.

Thin film transistors useful in forming ac-powered logic circuitry, asdescribed herein, may take a variety of forms and may be manufacturedusing various manufacturing processes. For example, the thin filmtransistors may include organic semiconducting material, inorganicsemiconducting material, or a combination of both. For someapplications, organic and inorganic semiconducting materials can be usedto form CMOS thin film transistor circuitry. Thin film transistorsuseful in forming ac-powered logic circuitry as described herein mayinclude, without limitation, thin film transistors manufacturedaccording to the techniques described in U.S. Pat. No. 6,433,359; U.S.patent application Ser. No. 10/012,654, filed Nov. 2, 2001; U.S. patentapplication Ser. No. 10/012,655, filed Nov. 5, 2001; U.S. patentapplication Ser. Nos. 10/076,174, 10/076,005, and 10/076,003, all filedon Feb. 14, 2002; and U.S. patent application Ser. No. 10/094,007, filedMar. 7, 2002; the entire content of each being incorporated herein byreference.

Various modification may be made without departing from the spirit andscope of the invention. These and other embodiments are within the scopeof the following claims.

1. A radio frequency identification (RFID) tag comprising: a logic gateformed by at least a first transistor and a second transistor; and aradio frequency converter that converts RF energy to alternating current(ac) power, and directly powers the logic gate with the ac power.
 2. TheRFID tag of claim 1, wherein the logic gate includes one of an inverter,a NOR gate, and a NAND gate.
 3. The RFID tag of claim 1, wherein thelogic gate forms an analog amplifier.
 4. The RFID tag of claim 1,further comprising a load capacitor coupled to an output of the logicgate.
 5. The RFID tag of claim 4, wherein the logic gate is a firstlogic gate and the circuit further includes a second logic gate, whereinan output of the first logic gate drives an input of the second logicgate, and wherein the load capacitor is formed at least in part by aninput capacitance of the second logic gate.
 6. The RFID tag of claim 1,wherein the RFID tag includes a series of inverter stages, the inverterstages being coupled to form at least part of a ring oscillator.
 7. TheRFID tag of claim 6, further comprising: a plurality of data lines; anda plurality of logic gates that selectively output data from the datalines in response to a clock signal generated by the ring oscillator. 8.The RFID tag of claim 7, wherein the transistors include a plurality ofthin film transistors arranged to form at least part of the logic gates.9. The RFID tag of claim 1, wherein the ac power waveform has a periodless than a propagation delay time of the logic gate.
 10. The RFID tagof claim 1, wherein at least one of the transistors is an organic thinfilm transistor.
 11. The RFID tag of claim 10, wherein at least one ofthe transistors is pentacene-based.
 12. The RFID tag of claim 1, whereinat least one of the transistors is amorphous silicon-based.
 13. The RFIDtag of claim 1, wherein the logic gate comprises a CMOS logic gate. 14.The RFID tag of claim 1, wherein the transistors are formed on aflexible substrate.
 15. The RFID tag of claim 1, wherein the firsttransistor is a load transistor and the second transistor is a drivetransistor, and wherein a ratio of a gate width to a gate length of theload transistor is greater than or equal to a ratio of a gate width to agate length of the drive transistor.
 16. A radio frequencyidentification (RFID) system comprising: an RFID tag including first andsecond transistors arranged to form a logic gate, a radio frequency (RF)converter that converts RF energy to alternating current (ac) power anddirectly powers the logic gate with the ac power, and a modulator thatconveys information; and an RFID reader that transmits the RF energy tothe RFID tag for conversion by the RF converter, and reads theinformation conveyed by the modulator.
 17. The system of claim 16,wherein the logic gate includes one of an inverter, a NOR gate, and aNAND gate.
 18. The system of claim 16, wherein the logic gate forms ananalog amplifier.
 19. The system of claim 16, further comprising a loadcapacitor coupled to an output of the logic gate.
 20. The system ofclaim 19, wherein the logic gate is a first logic gate and the circuitfurther includes a second logic gate, wherein an output of the firstlogic gate drives an input of the second logic gate, and wherein theload capacitor is formed at least in part by an input capacitance of thesecond logic gate.
 21. The system of claim 16, wherein the RFID tagincludes a series of inverter stages, the inverter stages being coupledto form at least part of a ring oscillator.
 22. The system of claim 21,further comprising: a plurality of data lines; and a plurality of logicgates that selectively output data from the data lines in response to aclock signal generated by the ring oscillator.
 23. The system of claim22, wherein the transistors include a plurality of thin film transistorsarranged to form at least part of the logic gates.
 24. The system ofclaim 16, wherein the ac power waveform has a period less than apropagation delay time of the logic gate.
 25. The system of claim 16,wherein at least one of the transistors is an organic thin filmtransistor.
 26. The system of claim 25, wherein at least one of thetransistors is pentacene-based.
 27. The system of claim 16, wherein atleast one of the transistors is amorphous silicon-based.
 28. The systemof claim 16, wherein the logic gate comprises a CMOS logic gate.
 29. Thesystem of claim 16, wherein the transistors are formed on a flexiblesubstrate.
 30. The system of claim 16, wherein the first transistor is aload transistor and the second transistor is a drive transistor, andwherein a ratio of a gate width to a gate length of the load transistoris greater than or equal to a ratio of a gate width to a gate length ofthe drive transistor.
 31. A ring oscillator circuit comprising: aplurality of transistors arranged to form a series of inverter stages,the inverter stages being coupled to form a ring oscillator; and analternating current (ac) source to directly power the inverter stages inthe ring oscillator with an ac power waveform.
 32. The ring oscillatorcircuit of claim 31, wherein at least one of the transistors ispentacene-based.
 33. The ring oscillator circuit of claim 31, whereinthe transistors are formed on a flexible substrate.